The invention pertains to aqueous dispersions of particles of polymers with a glass transition temperature gradient, and to a method for the preparation of the same.
Recent changes in the legislation concerning the emission of organic solvents have led to a growing interest in water borne coating systems for industrial applications at ambient temperature. Water borne coating systems have been used for a long time already in applications where the decorative aspects of the coating were more important than the protective properties. The aqueous polymer dispersions that are used as binders in these coatings are often prepared by means of the emulsion polymerization process. Often these binders are thermoplastic acrylic copolymers. The mechanical properties of these systems are related to the high molecular weights that can be obtained by the emulsion polymerization process. A serious drawback to the conventional emulsion polymerization process is the fact that substantial amounts of protective colloids and surfactants must be used. Conventional surfactants or emulsifiers and protective colloids are highly water sensitive and impart poor water resistance to coatings with binders thus prepared. Furthermore, conventional surfactants or emulsifiers and protective colloids often act as plasticizer for the polymers, resulting in reduced blocking resistance. Blocking refers to the tendency of painted surfaces to stick together when placed in contact with each other under pressure. It is important for a coating to have good resistance to blocking, especially when painted surfaces come into contact. The blocking resistance of a coating depends on the polymer""s hardness and on external factors such as pressure, temperature, humidity, layer thickness, and drying conditions.
Aqueous polymer dispersions that are free of conventional surfactants using the salt of a monoester of an unsaturated dicarboxylic acid which is copolymerized with the other comonomers are known, for instance from U.S. Pat. No. 4,089,828. These emulsions may be used in textile finishing, paper sizing, industrial finishes, clear films, and air-dry paints. However, although the blocking resistance of these dispersions is satisfactory, paint formulations based on these dispersions possess a volatile organic content (VOC) that is too high according to the latest environmental legislation. Furthermore, paints based on dispersions disclosed in the patent above will lack the balance between hardness (blocking resistance) and flexibility that is required for surface coatings intended for substrates with low dimensional stability such as wood or for substrates that are deformed after the coating has been applied (e.g. metal).
Several techniques are known to those skilled in the art to improve the balance between the required hardness and the flexibility. Step-wise or staged addition procedures such as described in U.S. Pat. No. 3,448,173 or U.S. Pat. No. 3,562,235, where different reactants are introduced into the polymerization reactor at different stages of the reaction, result in the formation of polymer particles possessing a morphology referred to as xe2x80x9ccore-shellxe2x80x9d. Other procedures that result in the formation of polymer particles with a non-homogeneous morphology are known, for instance from U.S. Pat. No. 3,804,881, where polymers are produced by continuously introducing a primary polymerizable feed composition from a primary feed source to a polymerization zone, which primary polymerizable feed composition is continually varying in compositional content during the continuous introduction. This variation is obtained by continuously adding to the primary feed source a different secondary polymerizable feed composition from a secondary feed source, so as to continually change the compositional content of the reactants in the primary feed source. Coating compositions formulated with polymer dispersions prepared according to the techniques described above may possess low VOC values and have a favorable balance between flexibility and blocking resistance, yet still contain substantial amounts of water-sensitive surfactants and/or protective colloids. The presence of these water-sensitive compounds in the final coating is undesirable, since it adversely influences properties such as water resistance and gloss retention upon weathering.
It has now been found that the aforementioned disadvantages can be circumvented by an aqueous dispersion of particles of a polymer having a functional group for cross-linking, which is obtainable by a free radical emulsion polymerization of at least two different ethylenically unsaturated monomers, a surfactant of the formula M+.xe2x88x92OOCxe2x80x94CHxe2x95x90CHCOOR, wherein R is C(6-22) alkyl and M+ is Na+, K+, Li+, NH4+, or a protonated or quaternary amine, and a co-reactive compound having at least two groups capable of reacting with the functional group for cross-linking, characterized in that the glass transition temperature (Tg) of the polymer at the center of the particle is different from the Tg of the polymer at the surface of the particle, the difference being at least 55xc2x0 C., and the Tg of the polymer having the highest Tg is at least 40xc2x0 C.
Coatings having low VOC using these dispersions as binder show very good flexibility while maintaining superior blocking resistance and gloss retention properties.
A key feature of the present invention is the difference between the Tg of the polymer at the center and the Tg of the polymer at the surface of the particle, although it is not important whether the polymer at the surface has the higher or lower Tg. In principle, two types of particles are envisaged, i.e. core-shell particles with two different polymers in the core and the shell, respectively, having different glass transition temperatures (the gradient thus being a discontinuous gradient), and particles where the composition continuously varies in compositional content from the center to the surface of the particle. The concentration and the Tg gradient may have a linear character, but they may also be convex or concave or have any other character. Particles of this latter type are preferred and can be made by the method of U.S. Pat. No. 3,804,881.
The temperature gradient from the center to the surface leads to glass transition temperatures differing by at least 55xc2x0 C., with the highest Tg being at least 40xc2x0 C. More preferably, the difference in Tg is 75xc2x0 C. and the highest Tg is at least 50xc2x0 C.
The ethylenically unsaturated emulsion-polymerizable monomers are selected from a monovinylidene aromatic monomer, an xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and combinations thereof.
Accordingly, preferred monovinylidene aromatic monomers include styrene, xcex1-methyl styrene, vinyl toluene, o-, m-, and p-methylstyrene, o-, m-, and p-ethylstyrene, and combinations thereof.
Preferred xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomers include methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, butyl methacrylate, tert-butyl acrylate, 2-ethyl hexyl acrylate, and combinations thereof.
Preferred vinyl ester monomers include vinyl acetate, vinyl esters of versatic acid such as the monomers sold by Shell Chemicals as VEOVA(trademark) 9, 10, and 11, acrylonitrile, and combinations thereof.
The monomeric mixture may, if desired, include a chain transfer agent (or more than one chain transfer agent), as well as minor amounts of monomers having more than one ethylenically unsaturated bond.
The polymers have pendant groups, preferably comprising a moiety selected from xe2x80x94CORxe2x80x2, xe2x80x94COxe2x80x94CH2xe2x80x94COCH3 and xe2x80x94CH2OH, wherein Rxe2x80x2 is H or C(1-4) alkyl. These pending groups make the polymer cross-linkable with co-reactive compounds that are added to the polymer dispersion after the emulsion polymerization process.
The pendant groups can be obtained from the copolymerization of the ethylenically unsaturated monomer with a different ethylenically unsaturated monomer. Examples of such monomers are acrylate esters having a hydroxy group such as hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate. Also monomers having latent hydroxy groups such as glycidyl methacrylate can be used. Particularly preferred monomers are ketone-functional monomers such as the acetoacetoxy esters of hydroxyalkyl acrylates and methacrylates such as acetoacetoxyethyl methacrylate, and also keto-containing amides such as diacetoneacrylamide
The choice of the co-reactive compound that is added to the polymer dispersion and that can react with the pendant functional group of the polymer depends on the pendant group. This compound may be either polymer or a low-molecular weight compound. In order to affect cross-linking, the co-reactive compound must possess at least two co-reactive groups.
Examples of suitable co-reactive groups for the given pendant functional groups are known to those skilled in the art. Non-limiting examples are given in Table I.
The polymers optionally contain further functional groups that are used to impart certain properties to the polymer dispersion, such as stability, or to the coating formulated with the polymer dispersion, such as adhesion. The stability of the polymer dispersion can be improved by the use of comonomers with hydrophilic groups such as acid or amide functions.
Typically, the acid-bearing comonomers are olefinically unsaturated carboxyl-functional monomers such as mono carboxyl-functional acrylic monomers and olefinically unsaturated dicarboxyl-bearing monomers. Examples include acrylic acid, methacrylic acid, and itaconic acid. Sulfonic acid-bearing monomers may also be used, such as styrene p-sulfonic acid. An acid-bearing monomer may be polymerized as the free acid or as a salt, e.g. the NH4 or alkali metal salts of ethylmethacrylate-2-sulfonic acid or 2-acrylamido-2-methylpropane sulfonic acid, or the corresponding free acids. Amide-functional comonomers such as acrylamide and methacrylamide may also be used.
Other functional monomers that can be included to improve the adhesion of coatings containing the polymer dispersion comprise tertiary amino or ethylene ureido-functional monomers such as dimethylaminoethyl methacrylate and N-(2-methacryloyloxethyl)ethylene urea.
The invention also pertains to a method for preparing the aqueous dispersion, characterized in that at least two different ethylenically unsaturated monomers and a surfactant of the formula M+.xe2x88x92OOCxe2x80x94CHxe2x95x90CHCOOR, wherein R is C(6-22) alkyl and M+ is Na+, K+, Li+, NH4+, or a protonated or quaternary amine are subjected to a free radical emulsion polymerization reaction to obtain particles of a polymer with a functional group for cross-linking, wherein the difference between the Tg of the polymer at the center of the particle and of the polymer at the surface of the particle is at least 550xc2x0 C., and the highest Tg is at least 40xc2x0 C., after which a co-reactive compound with at least two groups capable of reacting with the functional group for cross-linking is added.
In a preferred embodiment a gradient of the Tg is obtained by continuously adding one or more ethylenically unsaturated monomers (I) to one or more ethylenically unsaturated monomers (II), at least one of which is different from the one or more monomers (I), with continuous feeding of the mixture of monomers to a reactor where the mixture is subjected to a free radical emulsion polymerization reaction, in order to obtain particles of a polymer having a functional group for cross-linking and a continuous Tg gradient, wherein the difference between the Tg of the polymer at the center of the particle and the polymer at the surface of the particle is at least 55xc2x0 C., and the highest Tg is at least 40xc2x0 C.
In a preferred embodiment the functional group for cross-linking with the co-reactive compound is attached to an ethylenically unsaturated monomer in the monomer or the mixture of monomers (I) or (II), which has the lowest Tg according to the Fox equation.
The Fox equation is a well-known equation described in Bulletin of American Physics Society, Vol. 1, Issue 3, page 123 (1956), and is used to calculate the Tg of the acrylic polymer that can be obtained from a mixture of acrylic monomers by using the weight fractions and the Tg""s of said monomers.
The particles can also be of the core-shell type, wherein the difference between the Tg of the polymer in the core and the polymer of the shell is at least 55xc2x0 C., and wherein the highest Tg is at least 40xc2x0 C. Methods for making core-shell polymers are well-known in the art, and any one of the available standard methods for making such particles can be employed.
The dispersions of the invention are suitable to be used in different types of coating compositions (e.g. protective, decorative, or adhesive) on a variety of substrates. For such purposes the dispersions may be combined or formulated with other additives or components, such as defoamers, rheology control agents, thickeners, dispersing and stabilizing agents (usually surfactants), wetting agents, fillers, extenders, fungicides, bactericides, coalescing solvents, wetting solvents, plasticizers, anti-freeze agents, waxes, and pigments.
Substrates to which the coating compositions may be applied include wood, metal, paper, and plastic substrates. The coating compositions may be applied to the substrate by any conventional method such as by brushing, dipping, flow coating, roller coating, and spraying. The dispersions of the invention are particularly suitable for opaque or translucent wood coatings and stains.
Until their use is desired, the coating compositions formulated with the dispersions of this invention can be stored at room temperature in a suitable container. When use is desired, the coating composition is applied to a suitable substrate. After application the volatile components in the coating composition will evaporate. The temperature at which the applied coating is dried can vary from ambient temperature to more elevated temperatures. The maximum temperature often is related to the nature of the substrate. The evaporation and drying of the applied coating will allow the cross-linking reaction between the functional groups on the polymer and the co-reactive compound to take place.
The invention is further illustrated by the following examples.